Components

An Ext JS application's UI is made up of one or many widgets called Components. All Components are subclasses of the Ext.Component class
which allows them to participate in automated lifecycle management including instantiation, rendering, sizing and positioning, and destruction.
Ext JS provides a wide range of useful Components out of the box, and any Component can easily be extended to create a customized Component.

The Component Hierarchy

A Container is a special type of Component that can contain other Components. A typical application is made up of many nested Components in a tree-like
structure that is referred to as the Component hierarchy. Containers are responsible for managing the Component lifecycle of their children, which includes creation, rendering,
sizing and positioning, and destruction. A typical application's Component hierarchy starts with a Viewport at the top,
which has other Containers and/or Components nested within it:

Child Components are added to a Container using the Container's items configuration property. This example uses Ext.create
to instantiate two Panels, then adds those Panels as child Components of a Viewport:

See the Container Example for a working demo showing how to add Components to a Container using the items configuration.

XTypes and Lazy Instantiation

Every Component has a symbolic name called an xtype. For example Ext.panel.Panel has an xtype of 'panel'.
The xtypes for all Components are listed in the API Docs for Component.
The above example showed how to add already instantiated Components to a Container.
In a large application, however, this is not ideal since not all of the Components need to be instantiated right away,
and some Components might never be instantiated depending on how the application is used. For example an application that uses a Tab Panel
will only need the contents of each tab to be rendered if and when each tab is clicked on by the user. This is where xtypes come in handy
by allowing a Container's children to be configured up front, but not instantiated until the Container determines it is necessary.

The following example code demonstrates lazy instantiation and rendering of a Container's Child components using a Tab Panel.
Each tab has an event listener that displays an alert when the tab is rendered.

Floating Components

Floating Component are positioned outside of the document flow using CSS absolute positioning, and do not participate in their Containers' layout.
Some Components such as Windows are floating by default, but any Component can be made floating using the floating configuration.

The above code instantiates a Panel but does not render it. Normally a Component either has a renderTo
configuration specified, or is added as a child Component of a Container, but in the case of floating Components neither of these is needed.
Floating Components are automatically rendered to the document body the first time their show method is called:

panel.show(); // render and show the floating panel

Here are a few other configurations and methods to make note of related to floating components:

draggable - enables dragging of a floating Component around the screen.

Creating Custom Components

Composition or Extension

When creating a new UI class, the decision must be made whether that class should own an instance of a Component, or to extend that Component.

It is recommended to extend the nearest base class to the functionality required. This is because of the automated lifecycle management Ext JS provides which
includes automated rendering when needed, automatic sizing and positioning of Components when managed by an appropriate layout manager,
and automated destruction on removal from a Container.

It is easier to write a new class which is a Component and can take its place in the Component hierarchy rather than a new class which has an Ext JS Component,
and then has to render and manage it from outside.

Subclassing

The Class System makes it easy to extend existing Components. The following example creates a subclass of Ext.Component without
adding any additional functionality:

Template Methods

Ext JS uses the Template method pattern to delegate to subclasses, behavior which is specific only to that subclass.

The meaning of this is that each class in the inheritance chain may "contribute" an extra piece of logic to certain phases in the Component's lifecycle.
Each class implements its own special behavior while allowing the other classes in the inheritance chain to continue to contribute their own logic.

An example is the render function. render is a private method defined in Component's superclass,
AbstractComponent that is responsible for initiating the rendering phase of the Component lifecycle.
render must not be overridden, but it calls onRender during processing to allow the subclass implementor to add an onRender
method to perform class-specific processing. Every onRender method must call its superclass' onRender method before "contributing" its extra logic.

The diagram below illustrates the functioning of the onRender template method.

The render method is called (This is done by a Container’s layout manager). This method may not be overridden and is implemented by the Ext base class.
It calls this.onRender which is the implementation within the current subclass (if implemented).
This calls the superclass version which calls its superclass version etc. Eventually, each class has contributed its functionality, and control returns to the render function.

Here is an example of a Component subclass that implements the onRender method:

It is important to note that many of the template methods also have a corresponding event. For example the render
event is fired after the Component is rendered. When subclassing, however, it is it is essential to use template methods to perform class logic at
important phases in the lifecycle and not events. Events may be programmatically suspended, or may be stopped by a handler.

Below are the template methods that can be implemented by subclasses of Component:

initComponent
This method is invoked by the constructor. It is used to initialize data, set up configurations, and attach event handlers.

beforeShow
This method is invoked before the Component is shown.

onShow
Allows addition of behavior to the show operation. After calling the superclass’s onShow, the Component will be visible.

afterShow
This method is invoked after the Component is shown.

onShowComplete
This method is invoked after the afterShow method is complete

onHide
Allows addition of behavior to the hide operation. After calling the superclass’s onHide, the Component will be hidden.

afterHide
This method is invoked after the Component has been hidden

onRender
Allows addition of behavior to the rendering phase.

afterRender
Allows addition of behavior after rendering is complete. At this stage the Component’s Element will have been styled according to the configuration,
will have had any configured CSS class names added, and will be in the configured visibility and the configured enable state.

onEnable
Allows addition of behavior to the enable operation. After calling the superclass’s onEnable, the Component will be enabled.

onDisable
Allows addition of behavior to the disable operation. After calling the superclass’s onDisable, the Component will be disabled.

onAdded
Allows addition of behavior when a Component is added to a Container. At this stage, the Component is in the parent Container's collection of child items.
After calling the superclass's onAdded, the ownerCt reference will be present, and if configured with a ref, the refOwner will be set.

onRemoved
Allows addition of behavior when a Component is removed from its parent Container. At this stage, the Component has been removed from its parent Container's
collection of child items, but has not been destroyed (It will be destroyed if the parent Container's autoDestroy is true, or if the remove call was passed a truthy second parameter).
After calling the superclass's onRemoved, the ownerCt and the refOwner will not be present.

onResize
Allows addition of behavior to the resize operation.

onPosition
Allows addition of behavior to the position operation.

onDestroy
Allows addition of behavior to the destroy operation. After calling the superclass’s onDestroy, the Component will be destroyed.

beforeDestroy
This method is invoked before the Component is destroyed.

afterSetPosition
This method is invoked after the Components position has been set.

afterComponentLayout
This method is invoked after the Component is laid out.

beforeComponentLayout
This method is invoked before the Component is laid out.

Which Class To Extend

Choosing the best class to extend is mainly a matter of efficiency, and which capabilities the base class must provide.
There has been a tendency to always extend Ext.Panel whenever any set of UI Components needs to be rendered and managed.

The Panel class has many capabilities:

Border

Header

Header tools

Footer

Footer buttons

Top toolbar

Bottom toolbar

Containing and managing child Components

If these are not needed, then using a Panel is a waste of resources.

Component

If the required UI Component does not need to contain any other Components, that is, if it just to encapsulate some form of HTML which performs the requirements,
then extending Ext.Component is appropriate. For example, the following class is a Component that wraps an HTML image element, and allows setting
and getting of the image's src attribute. It also fires a load event when the image is loaded:

See the Managed Image Example for a working demo. This example is for demonstration purposes only -
the Ext.Img class should be used for managing images in a real world application.

Container

If the required UI Component is to contain other Components, but does not need any of the previously mentioned additional capabilities of a Panel,
then Ext.container.Container is the appropriate class to extend. At the Container level, it is important to remember which Layout
is to be used to render and manage child Components.

Containers have the following additional template methods:

onBeforeAdd
This method is invoked before adding a new child Component. It is passed the new Component, and may be used to modify the Component, or prepare the Container in some way. Returning false aborts the add operation.

onAdd
This method is invoked after a new Component has been added. It is passed the Component which has been added. This method may be used to update any internal structure which may depend upon the state of the child items.

onRemove
This method is invoked after a new Component has been removed. It is passed the Component which has been removed. This method may be used to update any internal structure which may depend upon the state of the child items.

beforeLayout
This method is invoked before the Container has laid out (and rendered if necessary) its child Components.

afterLayout
This method is invoked after the Container has laid out (and rendered if necessary) its child Components.

Panel

If the required UI Component must have a header, footer, or toolbars, then Ext.Panel is the appropriate class to extend.

Important: A Panel is a Container. It is important to remember which Layout is to be used to render and manage child Components.

Classes which extend Ext.Panel are usually highly application-specific and are generally used to aggregate other UI Components
(Usually Containers, or form Fields) in a configured layout, and provide means to operate on the contained Components by means
of controls in the tbar and the bbar.

Panels have the following additional template methods:

afterCollapse
This method is invoked after the Panel is Collapsed.

afterExpand
This method is invoked after the Panel is expanded

onDockedAdd
This method is invoked after a docked item is added to the Panel

onDockedRemove
This method is invoked after a docked item is removed from the Panel